Photo-solubilizable compositions comprising an o-quinonediazide compound and a novolak resin provide such an excellent performance that they have been widely used industrially for producing lithographic printing plates or photoresists. However, the photo-solubilizable composition is not desirable because the o-quinonediazide compound, which itself appears yellowish, is photodecomposed by exposure to a colorless compound to give a visible image with a low contrast. Further, the resulting image cannot be visually recognized under a yellow safe lamp employed in actual exposing procedure, and such a composition provides such a low image contrast after development that inking is required in plateinspecting procedures such as erasing procedure.
With respect to this point, it is known to add, to the photo-solubilizable composition, a compound which undergoes photolysis upon exposure to produce an acidic substance and an organic dye which mutually acts with the photolysis-produced acidic substance to undergo change in color. Useful substances capable of producing an acidic substance by photolysis are disclosed in the following: U.S. Pat. No. 3,969,118 proposes o-naphthoquinonediazide-4-sulfonic acid halides, U.S. Pat. No. 4,160,671 proposes trihalomethyl-2-pyrrones and trihalomethyltriazines, British Pat. No. 2,038,801 proposes various o-naphthoquinonediazide compounds, and U.S. Pat. No. 4,279,982 proposes 2-trihalomethyl-5-aryl-1,3,4-oxadiazole compounds and, as the organic dye which mutually acts with photolysis products of these substances, diphenylmethane type, triphenylmethane type, thiazine type, oxazine type, phenazine type, xanthene type, anthraquinone type, iminonaphthoquinone type, and azomethine type dyes are shown.
Examples of these dyes are: Brilliant Green, Eosine, Ethyl Violet, Erythrosine B, Methyl Green, Crystal Violet, Basic Fuchsine, phenolphthalein, 1,3-diphenyltriazine, Alizarine Red S, Thymolphthalein, Methyl Violet 2B, Quinaldine Red, Rose Bengale, Metanil Yellow, Thymolsulfophthalein, Xylenol Blue, Methyl Orange, Orange IV, diphenylthiocarbazone, 2,7-dichlorofluoresceine, Paramethyl Red, Congo Red, Benzopurpurin 4B, .alpha.-Naphthyl Red, Nile Blue 2B, Nile Blue A, phenacetaline, Methyl Violet, Malachite Green, Parafuchsine, Oil Blue #603 (made by Orient Kagaku Kogyo Kabushiki Kaisha), Oil Pink #312 (made by Orient Kagaku Kogyo Kabushiki Kaisha), Oil Red 5B (made by Orient Kagaku Kogyo Kabushiki Kaisha), Oil Scarlet #308 (made by Orient Kagaku Kogyo Kabushiki Kaisha), Oil Red OG (made by Orient Kagaku Kogyo Kabushiki Kaisha), Oil Red RR (made by Orient Kagaku Kogyo Kabushiki Kaisha), Oil Green #502 (made by Orient Kagaku Kogyo Kabushiki Kaisha), Spiron Red BEH Special (made by Hodogaya Chemical Co., Ltd.), m-cresol purple, Cresol Red, Rhodamine B, Rhodamine 6G, Fast Acid Violet R, Sulforhodamine B, Auramine, 4-p-diethylaminophenyliminonaphthoquinone, 2-carboxyanilino-4-p-diethylaminophenyliminonaphthoquinone, 2-carbostearylamino-4-p-dihydroxyethylaminophenyliminonaphthoquinone, p-methoxybenzoyl-p'-diethylamino-o'-methylphenyliminoacetanilide, cyano-p-diethylaminophenyliminoacetanilide, 1-phenyl-3-methyl-4-p-diethylaminophenylimino-5-pyrazolone, 1-.beta.-naphthyl-4-p-diethylaminophenylimino-5-pyrazolone, etc.
In recent printing plates, previously grained and anodized aluminum supports have come into use for the purpose of attaining longer press life, high water retention, high color stain resistance, etc. However, when a photo-solubilizable composition comprising an o-quinonediazide compound and a novolak resin and containing the aforesaid dye capable of mutually acting with the photolysis product is coated on the anodized aluminum support, the following defects arise. That is, the added dye is so strongly adsorbed on an oxide film formed on the support that it cannot be completely removed after exposure and development processing, with part of it remaining as color stain, resulting in insufficient image contrast after development processing. Therefore, fine images, for example, several % minimum dots, film edge, image of refuse depositing on printer film upon printing, etc., become obscure. Particularly, film edge and deposited refuse form images unnecessary for printing, and hence they must be removed. However, special addition of the dye for raising image contrast is not effective due to the color stain, thus inking is required.
In addition, development processing is conducted in some cases with a fresh developing solution having high developing power, and in other cases with a fatigued developing solution having deteriorated developing power. In many cases, color stain tends to become stronger when development is conducted with a fatigued developing solution than when conducted with a fresh developing solution. With some dyes, however, both fresh developing solution and fatigued developing solution bring about strong color stain. Recently, development processing has been conducted in an automatic developing machine, but some users still conduct development processing manually. In manual development processing, areas where a developing solution is dropped are processed with a fresh developing solution, but areas where a developing solution is spread by sponge are developed with a fatigued developing solution. Accordingly, where a fatigued solution brings about stronger color stain than a fresh solution does, manual development processing results in uneven color stain.